Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process via cumene. This is a three-step process in which the first step involves alkylation of benzene with propylene in the presence of an acidic catalyst to produce cumene. The second step is oxidation, preferably aerobic oxidation, of the cumene to the corresponding cumene hydroperoxide. The third step is the cleavage of the cumene hydroperoxide in the presence of heterogeneous or homogenous catalysts into equimolar amounts of phenol and acetone, a co-product. However, the world demand for phenol is growing more rapidly than that for the acetone co-product. In addition, due to developing shortages in supply, the cost of propylene is likely to increase.
Thus, a process that avoids or reduces the use of propylene as a feed and coproduces higher ketones, rather than acetone, may be an attractive alternative route to the production of phenol. For example, there is a growing market for cyclohexanone, which is used as an industrial solvent, as an activator in oxidation reactions and in the production of adipic acid, cyclohexanone resins, cyclohexanone oxime, caprolactam, and nylon 6.
It is known that phenol and cyclohexanone can be co-produced by a variation of the Hock process in which cyclohexylbenzene is oxidized to obtain cyclohexylbenzene hydroperoxide and the hydroperoxide is decomposed in the presence of an acid catalyst to the desired phenol and cyclohexanone. Although various methods are available for the production of cyclohexylbenzene, a preferred route is via benzene hydroalkylation in which benzene is contacted with hydrogen in the presence of a catalyst such that part of the benzene is converted into cyclohexene which then reacts with the remaining benzene to produce the desired cyclohexylbenzene. One such method is disclosed in U.S. Pat. No. 6,037,513, in which the catalyst comprises a molecular sieve of the MCM-22 family and at least one hydrogenation metal selected from palladium, ruthenium, nickel, cobalt, and mixtures thereof. The '513 patent also discloses that the resultant cyclohexylbenzene can be oxidized to the corresponding hydroperoxide which is then decomposed to the desired phenol and cyclohexanone co-product in roughly equimolar amounts.
However, non-negligible amounts of by-products, including phenylcyclohexanols, are generated during the oxidation and/or cleavage steps. To improve product yields, these by-products are preferably converted to cyclohexylbenzene by a two stage process involving dehydration followed by hydrogenation. However, the catalysts and conversion conditions suitable for effecting the dehydration reaction are also prone to driving potential side reactions of the phenylcyclohexene intermediates as well as any residual cyclohexylbenzene or desired product (cyclohexanone and phenol) that may be present in the dehydration feed. Thus, cyclohexanone can undergo aldol condensation to heavy products and phenol can be alkylated with phenylcyclohexene and cyclohexylbenzene. In addition, the phenylcyclohexene intermediate can undergo reactions such as dimerization, alkylation with residual cyclohexylbenzene, isomerization to other products, such as methylcyclopentylbenzene, and hydride transfer to biphenyl. Not only do these side reactions involve potential loss of valuable product, but they can also lead to deactivation of the dehydration catalyst.